As described in U.S. Pat. No. 6,603,134 issued to Norman R. Wild and Paul M. Leavey, Jr. on Aug. 5, 2003, assigned to the assignee hereof and incorporated herein by reference, a system for detecting target threats is described in which a high-energy laser is scanned across an area on the ground and retroreflections from the human eye or the focal plane associated with an optical system are detected, thus to give an indication of the presence of the target within the scanned field.
As described in the Wild patent, Wild et al. were the first to discover that any kind of focusing device in combination with a surface exhibiting any degree of reflectivity and positioned near the focal plane of the device acts as a retroreflector. This applies not only to the human eye, but to any optical system used by a threat or target. A typical, though not exclusive list of examples includes night vision goggles, telescopic sights of any kind, ordinance-aiming devices using optics, tracking sensors or missile guidance systems employing focal plane arrays. It is noted that such systems involve augmented optical returns due to the fact that reflected light emanating from the focal plane is focused out of the optics as a collimated beam. The same optical behavior that transforms incoming light into an image at the focal plane guarantees that reflections at this surface are collimated into a beam and sent directly back to the source of the incident light.
In such active search systems, a laser pulse is scanned across a given area, usually from an aircraft onto the ground, and retroreflection of the type described in the Wild et al. patent is detected, thus to detect the presence of an enemy or non-cooperative threat or target. As will be appreciated, the collimated retroreflected beam will come back to the interrogating laser only a small percentage of the time due to the fact that it is focused in only one direction, necessitating a large number of sweeps or scans in order to make sure that if there is a threat in the search area it will be detected. This requires extended dwell times in order to execute the large number of sweeps required to ensure that if there is a threat in the search area, it will be detected. Thus, looking for retroreflections is at best problematical.
What makes such a system more problematical is the fact that most optical designers tilt the focal plane associated with the optical system with respect to the optical axis or boresight of the system. This can be done for a number of reasons. However, since the development of the Wild et al. optical detection system, there are those who have sought to thwart the system by tilting the focal plane such that incoming radiation from a search laser is directed off the optical axis and is not retroreflected out along the optical axis. Thus even a large number of laser sweeps can fail to detect a target of interest if such angled focal planes are employed.
It is noted that the focal plane need not be angled by a great amount in order for there to be no retroreflection or specular reflection out of the optical system. The term specular reflection is used herein to refer that the reflection is in accordance with the Law of Reflection; i.e. a specular reflection in which the angle of incidence equals the angle of reflection.
Thus there has been a question as to whether one could even detect a target in which no specular reflection reached the interrogating laser.
As mentioned above, the Wild et al. system, which relies upon retroreflection, also suffers from the problem that the optical axis of the target optic usually is not pointed in the direction of the interrogating laser. Even when the target uses scanning optics, the retroreflected or specularly reflected energy is not returned to the interrogating laser more than momentarily, for instance, less than 1% of the time. Thus, when using an active search system to scan a given area on the ground, the probability of detecting such a specular reflection is exceedingly low due to the small amount of time that the target optics are pointed at the interrogating laser.
More particularly, there are several factors that may have prevented the widespread use of the techniques discovered by Wild et al. in active search applications. In such active search systems, a laser pulse is scanned across a given area, usually from an aircraft onto the ground, and retro-reflection of the type described in the Wild et al. patent is detected, thereby revealing the presence of a potential enemy threat or non-cooperative target.
It is noted that Wild et al. focused on a particular subset of optical systems, and by doing so, made implicit assumptions about the nature of the retro-reflections being produced by their technique. Specifically, they describe providing persistent, high-amplitude returns. Persistence refers to the fact that the return is always produced when the optic is illuminated. High-amplitude refers to a reliance on direct specular reflection. The term specular as used herein denotes the portion of reflected energy that behaves according to the elementary law of reflection equating the incident and reflected angles.
By intentional design and or technological advances, the landscape has now broadened to include devices and conditions not considered by Wild et al. What follows are several of the practical limitations of such advances, as well as the manner in which the subject method overcomes them.
First among these are developments in optical design that suppress the retro-reflections from optical systems. Whether done intentionally to counter the effects discovered by Wild et al., or simply to solve unrelated engineering challenges, techniques such as focal plane tilting can dramatically reduce optically augmented returns. The result of these developments is a lower detection probability. This is turn translates into longer search times and/or higher laser power requirements for active search systems.
The second effect is the low probability of observing specular returns from scanner devices like line arrays, reticles and rosettes. As mentioned above, a Wild et al. system relies upon on persistent specular returns. Since scanning systems have a low temporal duty cycle, their returns are intermittent, providing large returns only momentarily during each scan pattern. Thus when using an active search system to scan a given patch of ground, the detection probability is low indeed due to the small amount of time that the target optics and interrogating laser are mutually co-aligned. Those familiar with analogous radar systems will recognize this effect as the scan-on-scan problem.
The third series of effects are environmental factors encountered in operational environments. Clear air turbulence, for example, further exacerbates the intermittent return problem. In fact, it can even make the specular returns from staring sensors fade in and out over time. This requires extended dwell times or an increased number of sweeps over the same area in order to ensure the detection of existing targets.
Finally, ground clutter is a problem and is a consequence of the preceding effects. The lowered, intermittent and generally degraded optical returns are forced to compete with the comparable returns from ground bounces. It is from such a cluttered background that optical signals must emerge if they are to be detected.
Other factors which have a bearing on the need for an improved active search system and a method for its use include the following: 1. The large power requirement for active search lasers used over long ranges and wide search areas; 2. The high noise levels of conventional detector arrays; 3. The poor match between detector array temporal characteristics and requirements for active search receivers; and 5. The difference between ground and tracking sensor returns.
These circumstances have led to the failure of previous active search systems intended to find passive tracking sensors. Furthermore, they have limited systems in development to using passive means such as missile launch detection to perform the initial acquisition. There is therefore a need for an improved active sensor and a method for its use that provides a solution to the problems of the prior art.